Manufacturing method of semiconductor device and semiconductor device

ABSTRACT

The present invention makes it possible to improve the accuracy of wet etching and miniaturize a semiconductor device in the case of specifying an active region of a vertical type power MOSFET formed over an SiC substrate by opening an insulating film over the substrate by the wet etching. After a silicon oxide film having a small film thickness and a polysilicon film having a film thickness larger than the silicon oxide film are formed in sequence over an epitaxial layer, the polysilicon film is opened by a dry etching method, successively the silicon oxide film is opened by a wet etching method, and thereby the upper surface of the epitaxial layer in an active region is exposed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2014-131943 filed onJun. 26, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND

The present invention relates: to a semiconductor device, and inparticular to a technology effectively applicable to a semiconductordevice including a silicon carbide substrate, and a manufacturing methodthereof.

A semiconductor power device is required to have a low on resistance anda low switching loss in addition to a high withstanding voltage but theperformance of a silicon (Si) power device that currently is themainstream is coming close to its theoretical limits. Silicon carbide(SiC) has a breakdown field strength about one digit larger than Si andhence the resistance of a device can be reduced theoretically by notless than three digits by reducing the thickness of a drift layer toretain withstanding voltage to about one tenth and increasing animpurity concentration by about a hundred times. Further, SiC has a bandgap about three times larger than Si and hence can withstand hightemperature operation and an SiC semiconductor device is expected tohave performance exceeding an Si semiconductor device.

With attention focused on the advantages of SiC, the research anddevelopment of a DMOS (Double-Diffused MOSFET) as a power MOSFET (MetalOxide Semiconductor Field Effect Transistor) of a high withstandingvoltage has been advanced.

An example of a manufacturing method of a DMOS is described in PatentLiterature 1 (Japanese Unexamined Patent Application Publication No.2008-227172). It describes that a stepped section that is an insulatingfilm having a film thickness larger than a gate insulating film isformed beside the gate insulating film by a thermal oxidization methodby making use of the accelerated oxidation characteristic of anamorphous layer as a substrate.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2008-227172

SUMMARY

In a DMOS including a gate insulating film and a gate electrode stackedover a substrate, it is conceivable to form an insulating film having afilm thickness larger than the gate insulating film and playing the roleof element isolation and the like (hereunder merely called a fieldinsulating film) over the substrate in a field region beside the gateinsulating film. Here, since the thermal oxidation rate of SiC isextremely slower than that of Si, it is difficult to form a fieldinsulating film having a LOCOS (Local Oxidation of Silicon) structureand a sufficiently large thickness. To cope with that, it is conceivableto form a thin gate insulating film in an active region and a thickfield insulating film separately by depositing an insulating film havinga sufficiently large film thickness over a substrate, successivelyremoving the insulating film selectively by wet etching, and thusforming the gate insulating film.

By a patterning method using wet etching however, the accuracy of thepatterning is low and a tapered opening is formed in a processedinsulating film. As a result, the processing of a relatively thickinsulating film by wet etching as stated above causes a semiconductordevice to be hardly miniaturized.

Other objects and novel features will be obvious from the descriptionsand attached drawings in the present specification.

The outlines of representative embodiments in the embodiments disclosedin the present application are briefly explained as follows.

A manufacturing method of a semiconductor device according to anembodiment is to specify an active region of a MOSFET by forming asilicon oxide film having a small film thickness and a polysilicon filmhaving a film thickness larger than the silicon oxide film in sequenceover an SiC substrate, thereafter opening the polysilicon film by a dryetching method, and successively opening the silicon oxide film by a wetetching method.

Further, a semiconductor device according to an embodiment is asemiconductor device having a source region and a channel region formedside-by-side over the upper surface of an epitaxial layer over an SiCsubstrate and a gate electrode formed over the channel region with agate insulating film interposed, a part of the gate electrode beingembedded right under an eaves-like protruding sidewall of an insulatingfilm formed beside the gate electrode.

According to an embodiment disclosed in the present application, it ispossible to improve the performance of a semiconductor device. Inparticular, it is possible to materialize the miniaturization of asemiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a manufacturing method of asemiconductor device according to Embodiment 1 of the present invention.

FIG. 2 is a sectional view showing a manufacturing method succeedingFIG. 1 of the semiconductor device.

FIG. 3 is a sectional view showing a manufacturing method succeedingFIG. 2 of the semiconductor device.

FIG. 4 is a sectional view showing a manufacturing method succeedingFIG. 3 of the semiconductor device.

FIG. 5 is a sectional view showing a manufacturing method succeedingFIG. 4 of the semiconductor device.

FIG. 6 is a sectional view showing a manufacturing method succeedingFIG. 5 of the semiconductor device.

FIG. 7 is a sectional view showing a manufacturing method succeedingFIG. 6 of the semiconductor device.

FIG. 8 is a sectional view showing a manufacturing method succeedingFIG. 7 of the semiconductor device.

FIG. 9 is a sectional view showing a manufacturing method succeedingFIG. 8 of the semiconductor device.

FIG. 10 is a sectional view showing a manufacturing method succeedingFIG. 9 of the semiconductor device.

FIG. 11 is a sectional view showing a manufacturing method succeedingFIG. 10 of the semiconductor device.

FIG. 12 is a sectional view showing a manufacturing method succeedingFIG. 11 of the semiconductor device.

FIG. 13 is a sectional view showing a manufacturing method succeedingFIG. 12 of the semiconductor device.

FIG. 14 shows a sectional view and a planar layout showing asemiconductor device according to Embodiment 1 of the present invention.

FIG. 15 is a sectional view enlargedly showing a semiconductor deviceaccording to Embodiment 1 of the present invention.

FIG. 16 is a sectional view enlargedly showing a semiconductor deviceaccording to Embodiment 1 of the present invention.

FIG. 17 is a sectional view showing a manufacturing method of asemiconductor device according to Embodiment 2 of the present invention.

FIG. 18 is a sectional view showing a manufacturing method succeedingFIG. 17 of the semiconductor device.

FIG. 19 is a sectional view showing a manufacturing method succeedingFIG. 18 of the semiconductor device.

FIG. 20 is a sectional view showing a manufacturing method succeedingFIG. 19 of the semiconductor device.

FIG. 21 is a sectional view showing a manufacturing method of asemiconductor device as a comparative example.

FIG. 22 is a sectional view showing a manufacturing method succeedingFIG. 21 of the semiconductor device.

FIG. 23 is a sectional view showing a manufacturing method succeedingFIG. 22 of the semiconductor device.

FIG. 24 is a sectional view showing a manufacturing method succeedingFIG. 23 of the semiconductor device.

DETAILED DESCRIPTION

Embodiments according to the present invention are hereunder explainedin detail in reference to the drawings. Here, in all the drawings forexplaining the embodiments, members having identical function arerepresented with an identical code and are not explained repeatedly.Further, in the following embodiments, identical or similar parts arenot explained repeatedly in principle except when particularly needed.Furthermore, in the drawings for explaining the embodiments, hatchingmay sometimes be applied even in a plan view, a perspective view, oranother view in order to make a configuration easy to understand.

In addition, the code “−” or “+” represents a relative concentration ofimpurities having an conductivity type of an n-type or a p-type and, inthe case of n-type impurities for example, the impurity concentrationincreases in the order of “n⁻⁻”, “n⁻”, “n”, “n⁺”, and then “n⁺⁺”.

Meanwhile, in the present application, a substrate mainly containingsilicon carbide (SiC) is merely called an SiC substrate. Further, in thepresent application, an SiC substrate and an epitaxial layer formedthereover may be combined and called a substrate in some cases.Furthermore, “an end” or “a terminal” cited in the present applicationmeans an end of a structure such as a film in the direction along themain surface of an SiC substrate. In addition, “a width” cited in thepresent application means a length from an end to the other end of astructure such as a film in the direction along the main surface of anSiC substrate.

Embodiment 1

A manufacturing method of a semiconductor device according to thepresent embodiment is explained in the order of the steps in referenceto FIGS. 1 to 13. FIGS. 1 to 13 are sectional views explaining themanufacturing steps of a semiconductor device according to the presentembodiment. In the sectional views shown in FIGS. 1 to 13, the region onthe left side in each of the sectional views is an element region 1Awhere a plurality of MOSFETs are formed and the region on the right sidein each of the sectional views is a termination region 1B that is aperipheral region of a semiconductor chip. That is, the left side ineach of the sectional views represents a region on the center side ofthe semiconductor chip formed through a relevant manufacturing step andthe right side in each of the sectional views represents a region thatcomes to be the peripheral section of the semiconductor chip.

Firstly, as shown in FIG. 1, an n⁺-type SiC substrate SB is provided.N-type impurities are introduced into the SiC substrate SB at arelatively high concentration. The n-type impurities comprise nitrogen(N) for example and the concentration of the n-type impurities is 1×10¹⁹cm⁻³ for example.

Successively, an epitaxial layer EP that is an n⁻-type semiconductorlayer comprising SiC is formed over the main surface of the SiCsubstrate SB by an epitaxial growth method. The epitaxial layer EPcontains n-type impurities at an impurity concentration lower than theSiC substrate SB. The impurity concentration of the epitaxial layer EPdepends on the rated withstanding voltage of an element and is 1×10¹⁶cm⁻³ for example. The epitaxial layer EP comes to be a route of electriccurrent flowing vertically in a MOSFET formed later. That is, theepitaxial layer EP is a layer including a drift layer of a semiconductordevice.

Successively, a patterned insulating film HM1 is formed over the uppersurface of the epitaxial layer EP. The insulating film HM1 is a filmintermittently exposing the upper surface of the epitaxial layer EP inthe element region 1A. The insulating film HM1 is comprised of SiO₂(silicon oxide) or the like for example and concretely is comprised of aTEOS (Tetra Ethyl Ortho Silicate) film or the like for example. Theinsulating film HM1 covers the most part of the upper surface of theepitaxial layer EP in the termination region 1B. The insulating film HM1is formed by patterning an insulating film formed over the epitaxiallayer EP by a CVD (Chemical Vapor Deposition) method by aphotolithography technology and an etching method for example.

Successively, ions of p-type impurities (for example aluminum (Al)) areimplanted into the epitaxial layer EP over which the insulating film HM1is formed. As a result, a plurality of body regions (channel regions) BRthat are p⁻-type semiconductor regions are formed side-by-side over theupper surface of the epitaxial layer EP in the element region 1A. Thedepth of the body regions BP from the upper surface of the epitaxiallayer EP, namely a junction depth, does not reach the lower surface ofthe epitaxial layer EP. At the ion implantation step, the insulatingfilm HM1 is used as a hard mask.

Successively, as shown in FIG. 2, a sidewall SW is formed self-alignedlyover each of the sidewalls of the insulating film HM1. The sidewalls SWare formed for example by forming a silicon oxide film over theepitaxial layer EP by a CVD method or the like, thereafter removingparts of the silicon oxide film by a dry etching method, therebyexposing the upper surface of the epitaxial layer EP, and thus leavingthe silicon oxide film which is in contact with the sidewalls of theinsulating film HM1. Here, it is estimated that the upper surface of theepitaxial layer EP at the sites exposed by the dry etching method isdamaged through the dry etching step but, since the exposed sites of theepitaxial layer EP are not the regions to be channels of MOSFETs, theperformance deterioration and the like of the MOSFETs are not concerned.

Successively, ions of n-type impurities (for example nitrogen (N)) areimplanted into the upper surface of the epitaxial layer EP with theinsulating film HM1 and the sidewalls SW used as masks. As a result, aplurality of source regions SR that are n⁺-type semiconductor regionsare formed over the upper surface of the epitaxial layer EP. Each of thesource regions SR is formed at the center section of the relevant bodyregion BR in a planar view. That is, at the upper surface of theepitaxial layer EP, the parts of the epitaxial layer EP at which thebody regions BR and the source regions SR are not formed exist betweenadjacent body regions BR, and each of the parts of the body regions BRhaving a width of about 0.5 μm is interposed between the relevant partof the epitaxial layer EP and the relevant source region SR. The depthof the source regions SR from the upper surface of the epitaxial layerEP is shallower than the depth at which the body regions BR are formed.

The body region BR formed on both the sides of each of the sourceregions SR at the upper surface of the epitaxial layer EP is a regionthat comes to be a channel that is an electric current route of a MOSFETthat will be formed later. Here, a plurality of hard masks covering theupper surfaces of a plurality of channel regions, namely a plurality ofsidewalls SW, are formed self-alignedly at equal intervals in order toequalize the widths (for example 0.5 μm) of the relevant channels with ahigh degree of accuracy. The n-type impurity concentration of the sourceregions SR is higher than the n-type impurity concentration of theepitaxial layer EP.

Successively, as shown in FIG. 3, a photoresist film PR1 to cover theinsulating film HM1 and the sidewalls SW and expose the center sectionof the upper surface of each of the source regions SR is formed over theepitaxial layer EP by a photolithography technology. The pattern of thephotoresist film PR1 exposes the upper surface of the epitaxial layer EPat positions apart from opposing sidewalls SW between adjacentinsulating films HM1 in the element region 1A.

Successively, ions of p-type impurities (for example aluminum (A)) areimplanted at a relatively high concentration into the upper surface ofthe epitaxial layer EP exposed from the photoresist film PR1. As aresult, a plurality of contact regions CR that are p⁺-type semiconductorregions are formed over the upper surface of the epitaxial layer EP inthe element region 1A. Each of the contact regions CR is formed at thecenter section of the relevant source region SR in a planar view, namelyat the center section of the relevant body region BR in a planar view.

The depth of the contact regions CR from the upper surface of theepitaxial layer EP is deeper than the depth at which the source regionsSR are formed. Although the structure of forming the contact regions CRso as to be shallower than the depth at which the body regions BR areformed is shown in the figure, the depth at which the contact regions CRare formed may be deeper than the depth at which the body regions BR areformed. Each of the contact regions CR is a region formed for thepurpose of electrically coupling the relevant body region BR with ametallic film (source electrode) that will be described later in orderto fix the body regions BR to a prescribed potential (source potential).That is, the p-type impurity concentration of the contact regions CR ishigher than the p-type impurity concentration of the body regions BR,and each of the contact regions CR and the relevant body region BR arein contact with each other.

Successively, as shown in FIG. 4, after the photoresist film PR1, theinsulating film HM1, and the sidewalls SW are removed, a photoresistfilm PR2 to cover the element region 1A and expose a part of thetermination region 1B is formed. Successively, ions of p-type impurities(for example aluminum (Al)) are implanted into the upper surface of theepitaxial layer EP at a relatively low concentration with thephotoresist film PR2 used as a mask. As a result, a p⁻⁻-typesemiconductor region TM is formed at the upper surface of the epitaxiallayer EP in the termination region 1B.

The semiconductor region TM is in contact with a body region BR and isformed closer to the periphery of the semiconductor chip that is to besubsequently formed than the body region BR. Further, the semiconductorregion TM is formed at a depth deeper than the body region BR.Otherwise, the depth of the semiconductor region TM may be comparablewith the depth of the body region BR. Further, the semiconductor regionTM does not reach the lower surface of the epitaxial layer EP.

In the termination region 1B, the body region BR and the semiconductorregion TM having an impurity concentration lower than the body region BRare formed in the manner of being aligned in sequence in the directionof coming closer to the periphery of the semiconductor chip that is tobe formed through a succeeding step. By forming such a JTE (JunctionTermination Extension) structure, it is possible to: mitigate theelectric field at the end of the semiconductor chip; and prevent thewithstanding voltage of the device from deteriorating by electric fieldconcentration.

Successively, after the photoresist film PR2 is removed, a carbon layeris formed so as to cover the SiC substrate SB and the epitaxial layer EPand thereafter the substrate including the SiC substrate SB and theepitaxial layer EP is activated by applying heat treatment at 1,700° C.to 1,800° C. The heat treatment is applied at a relatively hightemperature and hence is applied before a gate electrode is formed.Successively, the carbon layer is removed.

Successively, as shown in FIG. 5, a silicon oxide film IF1 20 nm inthickness for example is formed over the whole surface of the epitaxiallayer EP by a CVD method or the like for example. Successively, apolysilicon film PS1 100 nm in thickness for example is formed over thewhole surface of the silicon oxide film IF1 by a CVD method or the likefor example. The polysilicon film PS1 is comprised of a silicon (Si)film that is a semiconductor film having a higher oxidation rate thansilicon carbide (SiC).

Successively, as shown in FIG. 6, a pattern of a photoresist film PR3 isformed over the polysilicon film PS1 by a photolithography technology.The photoresist film PR3 is a resist pattern having openings right overthe regions between adjacent source regions SR in the element region 1A.In the present application, a region between source regions SR, in whichregion a gate electrode is to be formed over a channel region through agate insulating film in a succeeding step, is called an active region insome cases. Further, a region on both the sides interposing the activeregion, in which region an insulating film having a film thicknesslarger than the gate insulating film is formed beside the gateinsulating film, is called a field region (element isolation region) insome cases.

Successively, the upper surface of the polysilicon film PS1 is exposedat the bottoms of a plurality of grooves T1 by applying dry etching withthe photoresist film PR3 used as a mask, thus opening the polysiliconfilm PS1, and thereby forming the grooves T1. On this occasion, in orderto prevent the upper surface of the epitaxial layer EP including thechannel regions from being damaged by the dry etching, the upper surfaceof the epitaxial layer EP is not exposed at the dry etching step. Thatis, the silicon oxide film IF1 that is an insulating film is used as anetching stopper film.

Successively, as shown in FIG. 7, after the photoresist film PR3 isremoved, parts of the silicon oxide film IF1 are removed and the uppersurface of the epitaxial layer EP in the active region is exposed byapplying wet etching with the polysilicon film PS1 used as a mask. As aresult, the upper surface of the body region BR that is to be thechannel region of each of the MOSFETs is exposed between the adjacentsource regions SR. That is, the upper surfaces of the two body regionsBR facing each other between adjacent source regions SR and theepitaxial layer EP between the two body regions BR are exposed.

Since not a dry etching method but a wet etching method is used here, itis possible to prevent the upper surfaces of the body regions BR thatare to be the channel regions from being damaged. As a result, it ispossible to prevent the performance deterioration such as the increaseof off-current in the MOSFETs from being caused.

Further, processing by a wet etching method is a processing methodhaving a relatively low positional accuracy of patterning. That is, whena mask pattern is formed at the upper part of a film to be etched andwet etching is applied, the arising problem is that the end of the filmretracts inside the end of the mask pattern and the magnitude of theretraction can hardly be controlled.

In the present embodiment in contrast, since the film thickness of thesilicon oxide film IF1 under the polysilicon film PS1 is about 20 nm andthus very small, it is possible to shorten the time of the wet etchingapplied for opening the silicon oxide film IF1 and exposing the uppersurface of the epitaxial layer EP. As a result, it is possible to reducethe magnitude of the retraction of the film to be etched caused by thewet etching method to the minimum and hence it is possible to improvethe processing accuracy of the etching. As a result, the width of theMOSFETs can be reduced and consequently the higher integration of theMOSFETs can be materialized.

Successively, as shown in FIG. 8, oxidation treatment is applied to theupper surface of the epitaxial layer EP exposed from the silicon oxidefilm IF1 at the bottoms of the grooves T1 and the polysilicon film PS1by applying heat treatment to the whole structure including theepitaxial layer EP and the polysilicon film PS1. As a result, a gateinsulating film GF is formed over the epitaxial layer EP exposed at theopenings of the silicon oxide film IF1, namely over the upper surfacesof the body regions BR that are the channel regions in the regionsadjacent to the source regions SR. Further, the gate insulating film GFis formed also over the epitaxial layer EP between the paired bodyregions BR exposed at the openings of the silicon oxide film IF1. Thefilm thickness of the gate insulating film GF is about 50 nm forexample.

Further, at the heat treatment step, the whole of the plural parts ofthe polysilicon film PS1 is oxidized and a field insulating film FI1comprising a silicon oxide film is formed. The field insulating film FI1is a film including an insulating film formed by oxidizing thepolysilicon film PS1 and the silicon oxide film IF1 formed under thepolysilicon films PS1 (refer to FIG. 7).

The polysilicon film PS1 is combined with oxygen (O) and comes to be asilicon oxide film through the heat treatment and the film thickness andthe width increase. As a result, the film thickness of the fieldinsulating film FI1 increases to about 250 nm for example. Consequently,over the epitaxial layer EP, the relatively thick field insulating filmFI1 formed in the field region and the relatively thin gate insulatingfilm GF formed in the active region are formed adjacently to and incontact with each other. Both the field insulating film FI1 and the gateinsulating film GF are comprised of silicon oxide films respectively,and are coupled with each other to be integrated, and configure an oxideinsulating film O1. A plurality of grooves T2 are formed at thepositions corresponding to the plural grooves T1 (refer to FIG. 7) inthe oxide insulating film O1, the grooves T2 do not pass through theoxide insulating film O1, the oxide insulating film O1 right under thegrooves T2 configures the gate insulating film GF, and the oxideinsulating film O1 of the sidewalls of the grooves T2 configure thefield insulating film FI1.

Here, by oxidizing the thick polysilicon film PS1 over the silicon oxidefilm IF1, the ends of the field insulating film FI1 are formed so thatthe upper parts may overhang on the active region side at the boundarybetween the active region and the field region. That is, the ends of thefield insulating film FI1 take an eaves-like shape over the uppersurfaces of the body regions BR in the active region.

In other words, the ends of the field insulating film FI1 are formedright over the relevant ends of the gate insulating film GF respectivelyso as to be separated from and overhang the gate insulating film GF. Asa result, the sidewalls of the field insulating film FI1 take aninversely-tapered shape. That is, the width of the field insulating filmFI1 in the lateral direction, namely in the direction along the mainsurface of the SiC substrate SB, increases from the lower surface towardthe upper surface. The ends of the upper surfaces of the fieldinsulating film FI1 are located right over the active region. That is,the minimum angle between the upper surface of the gate insulating filmGF or the upper surface of the epitaxial layer EP and the sidewalls ofthe field insulating film FI1 is an acute angle of less than 90 degrees.

Successively, as shown in FIG. 9, a polysilicon film PS2 and aninsulating film IF2 are formed over the oxide insulating film O1 by aCVD method for example. Although the figure shows that the filmthickness of the polysilicon film PS2 is smaller than the film thicknessof the insulating film IF2, it is also possible that the film thicknessof the polysilicon film PS2 is larger than the film thickness of theinsulating film IF2. N-type impurities (for example phosphorus (P)) areintroduced into the polysilicon film PS2.

Here, since the sidewalls of the field insulating film FI1 are formed inan eaves-like shape, a part of the polysilicon film PS2 deposited by aCVD method or the like is embedded into the region between the eavessections at the ends of the field insulating film FI1 and the gateinsulating film GF right under the eaves sections. As a result, the gateinsulating film GF, the polysilicon film PS2, the field insulating filmFI1, the polysilicon film PS2, and the insulating film IF2 are formed insequence right over the upper surfaces of the parts of the body regionsBR that are the channel regions.

Successively, as shown in FIG. 10, after the insulating film IF2 ispatterned by a photolithography technology and a dry etching method, dryetching is applied with the insulating film IF2 used as a hard mask andthus the polysilicon film PS2 is patterned. As a result, a gateelectrode GE comprising the polysilicon film PS2 is formed at pluralsites. A part of the gate electrode GE is formed so as to fill theinteriors of the grooves T2 right over the active region, namely rightover the gate insulating film GF. That is, the gate electrode GE isformed so as to fill the gap between opposing two sections of the fieldinsulating film FI1.

That is, the gate electrode GE being in contact with the upper surfaceof the gate insulating film GF is formed right over the body regions BR,namely the channel regions, exposed over the upper surface of theepitaxial layer EP adjacently to the source regions SR. Here, each ofthe ends of the upper surfaces of the field insulating film FI1 islocated right over the relevant end of the gate insulating film GF inthe active region and right over the gate electrode GE being in contactwith the upper surface of the relevant end of the gate insulating filmGF

Further, another part of the gate electrode GE is formed so as to be incontact with the upper surfaces of the field insulating film FI1adjacent to the active region outside the grooves T2. That is, the gateelectrode GE is formed from right over the one part to right over theother part of the two parts of the field insulating film FI1 facing eachother with the relevant groove T2 interposed. That is, the gateelectrode GE and the insulating film IF2 terminate right over the fieldinsulating film FI1. In other words, the gate insulating film GF, thegate electrode GE, the field insulating film FI1, and the gate electrodeGE exist in sequence right over the ends of the upper surfaces of thebody regions BR in the active region including the channel regions.

In this way, MOSFETs Q1 each of which comprises the gate electrode GE,the gate insulating film GF, the source region SR, the body region BRthat is the channel region, the epitaxial layer EP including the driftlayer, and the SiC substrate SB that is the drain region are formed. Ineach of the MOSFETs Q1, when a prescribed potential is applied to thegate electrode GE, a channel is formed in the body region BR right underthe gate electrode GE, thereby the electrons in the source region SRadjacent to the channel pass through the upper surface (channel) of thebody region BR, advance in the epitaxial layer EP in the verticaldirection, and flow into the SiC substrate SB, namely into the drainregion, and thus electric current flows in the direction opposite to theroute.

That is, each of the MOSFETs Q1 is a planar gate type vertical MOSFET.Each of the MOSFETs Q1 is an n-channel type field effect transistor anda power MOSFET having a structure called a DMOS (Double-DiffusedMOSFET).

When the polysilicon film PS2 is processed through the patterning step,the field insulating film FI1 has a film thickness larger than the gateinsulating film GF and hence it is unnecessary to concern that the oxideinsulating film O1 is penetrated by etching. Further, since the gateelectrode GE is processed with the insulating film IF2 used as a hardmask here, it is possible to improve the processing accuracy of the gateelectrode GE in the lateral direction.

Here, in the step for forming a MOSFET, it is conceivable to form aninsulating film having the same film thickness as the gate insulatingfilm even in the field region between elements without forming such athick insulating film as the field insulating film FI1. If we try toform the gate electrode by processing the polysilicon film over thefield region in such a structure however, in the case where the accuracyof etching is poor or the like, it is concerned that the insulating filmin the field region is penetrated or gets thin, thereby short circuit orwithstanding voltage drop is caused, and the MOSFET does not operatenormally. Further, if the insulating film is penetrated, the uppersurface of the epitaxial layer right under it is damaged.

In the present embodiment in contrast, by forming the field insulatingfilm FI1 having a film thickness larger than the gate insulating film GFbeside the gate insulating film GF, it is possible to facilitate theetching at the step for forming the gate electrode GE. Further, byforming the field insulating film FI1 having a large film thickness, itis possible to inhibit the electric field of a wire over the substratefrom being transmitted to a semiconductor element. Furthermore, whenelements (for example diodes or the like) other than MOSFETs that willbe described later are formed over the substrate, the field insulatingfilm FI1 having a comparatively large film thickness can be used as anelement isolation layer and hence it is possible to electricallyseparate the elements from each other.

Here, in order to form the oxide insulating film O1 having filmthicknesses different between the active region and the field region,the polysilicon film PS1 shown in FIG. 7 is formed and the uppersurfaces of the polysilicon film PS1 and the epitaxial layer EP areoxidized respectively through the step explained in reference to FIG. 8.Such a manufacturing method is a feasible method because of thesemiconductor device using not a bulk silicon (Si) but an SiC (siliconcarbide) substrate.

That is, if we try to oxidize the polysilicon film PS1 and the substrateupper surface respectively as stated above over a bulk silicon (Si)substrate, the oxidation rate of the bulk silicon is larger than that ofSiC and hence the whole of the polysilicon film PS1 having a large filmthickness cannot be oxidized even when oxidation treatment for forming athin gate insulating film over the substrate is applied. If the filmthickness of the polysilicon film PS1 is reduced and the film thicknessof the silicon oxide film IF1 under it is increased to cope with that,the problem of deteriorating the accuracy in the processing of thesilicon oxide film IF1 by a wet etching method (refer to FIG. 7) arises.

In the present embodiment in contrast, by using an SiC substrate havingan oxidation rate lower than a bulk silicon, it is possible to form thegate insulating film GF allowing the film thickness to be reduced inorder to operate a MOSFET and the field insulating film FI1 having afilm thickness necessary for facilitating the processing of the gateelectrode GE as shown in FIG. 8.

Successively, as shown in FIG. 11, an interlayer insulating film IF3comprising a silicon oxide film for example is formed over the epitaxiallayer EP so as to cover the oxide insulating film O1, the gate electrodeGE, and the insulating film IF2 by a CVD method or the like for example.Successively, by a photolithography technology and a dry etching method,parts of the interlayer insulating film IF3 and the oxide insulatingfilm O1 are removed respectively, the upper surface of the epitaxiallayer EP is exposed, and thereby a plurality of contact holes CH passingthrough the interlayer insulating film IF3 and the oxide insulating filmO1 are formed.

Each of the contact holes CH is a hole that is formed at a positionapart from the gate electrode GE between adjacent parts of the gateelectrode GE and exposes the relevant contact region CR and a part ofthe relevant source region SR around it over the upper surface of theepitaxial layer EP. Each of the contact holes CH is formed at the centersection of the relevant body region BR and the relevant source region SRin a planar view. Here, a contact hole to expose the upper surface ofthe gate electrode GE is also formed though it is not shown in thefigure.

Successively, although it is not shown in the figure, a silicide layeris formed over the upper surfaces of the contact regions CR, the sourceregions SR, and the gate electrode GE, those being exposed at the bottomsurfaces of the contact holes CH, by a known salicide technology. Thesilicide layer is comprised of NiSi (nickel silicide) for example.

Successively, as shown in FIG. 12, a metallic film M1 is formed over theSiC substrate SB by a sputtering method or the like. The metallic filmM1 is comprised of aluminum (Al) for example. The metallic film M1covers the upper surface of the interlayer insulating film IF3 and isembedded into the contact holes CH. Successively, the metallic film M1in the termination region 1B is removed by a photolithographytechnology.

The upper surface of the metallic film M1 is a pat section to which abonding wire or the like is coupled and the metallic film M1 embeddedinto each of the contact holes CH is a contact plug to supply aprescribed potential to the relevant MOSFET Q1. Each of the contactplugs is electrically coupled to the relevant contact region CR, therelevant source region SR, and the gate electrode GE through a silicidelayer (not shown in the figure). Here, the metallic film M1 (not shownin the figure) to supply a potential to the gate electrode GE, and themetallic film M1 to supply a potential to the contact regions CR and thesource regions SR are arranged in the manner of being separated andinsulated from each other.

Successively, as shown in FIG. 13, the upper part of the terminationregion 1B is covered with a protective film PI. The protective film PIis comprised of polyimide for example and covers the upper surface ofthe interlayer insulating film IF3 in the termination region 1B and theend of the metallic film M1 in the vicinity of the termination region1B. That is, the protective film PI exposes a pad section of themetallic film M1 (not shown in the figure) for supplying electricity tothe gate electrode GE and a pad section of the metallic film M1 forsupplying electricity to the source regions SR and the contact regionsCR.

Here, although it is not shown in the figure here, it is also possibleto form a drain region of an n⁺⁺-type over the back surface of the SiCsubstrate SB by an ion implantation method for example after the stepfor forming the protective film PI for example. Further, although it isalso not shown in the figure here, a silicide layer is formed so as tobe in contact with the back surface of the SiC substrate SB. Thesilicide layer is formed by depositing a metallic film comprising Ni(nickel) or the like over the back surface of the SiC substrate SB,thereafter heating the metallic film with a laser, and making it reactwith the SiC substrate SB for example. The reason why the heat treatmentis applied with a laser is to prevent a device such as a MOSFET Q1 frombeing overheated.

Successively, a metallic film M2 that is a drain electrode is formed soas to be in contact with the lower surface of the silicide layer. Themetallic film M2 is a film formed by stacking an aluminum (Al) film anda gold (Au) film in sequence for example. Successively, the SiCsubstrate SB is cut and individualized by dicing and thereby a pluralityof semiconductor chips are formed from one semiconductor wafer. In thisway, a semiconductor device according to the present embodimentincluding a semiconductor chip is completed.

Here, a sectional view and a planar layout of a semiconductor chipaccording to the present embodiment are shown in FIG. 14. A sectionalview of a semiconductor chip is shown on the upper side of FIG. 14 and aplanar layout of the semiconductor chip at a position corresponding tothe sectional view is shown under it. In the planar layout, in order tomake the figure easy to understand, hatching is applied to a gate pad GPthat is a pad section of a gate electrode GE and a source pad SP that isa pad section of a source electrode. Further, in the planar layout, theoutline of the gate electrode GE is shown with broken lines.

Here, in the planar layout, the outlines of the metallic film M1 areshown with heavy solid lines. The metallic film M1 to supply a potentialto the gate electrode GE is formed in the region where the gate pad GPin the figure is arranged and the metallic film M1 to supply a potentialto the source regions SR is a region surrounding the gate pad GP andincluding the source pad SP and is formed right over all of the contactregions CR and the source regions SR.

As shown in the planar layout of FIG. 14, in the element region 1A, aplurality of element units each of which contains a body region BR, asource region SR, and a contact region CR are arranged side-by side in amatrix. No element unit is formed however right under the gate pad GP.The semiconductor region TM in the termination region 1B is formedannularly along the peripheral section of the rectangular semiconductorchip so as to surround the element region 1A in a planar view. Here,although each of the element units is shown in an oblong shape in FIG.14, the shapes of the body region BR, the source region SR, and thecontact region CR configuring each of the element units may also besquares in a planar view.

The effects of the manufacturing method of a semiconductor deviceaccording to the present embodiment and the effects of a semiconductordevice according to the present embodiment are explained hereunder inreference to FIGS. 15 and 16 that are sectional views obtained byenlarging a part of the semiconductor device according to the presentembodiment and FIGS. 21 to 24 that are sectional views explaining thesteps for forming a semiconductor device according to a comparativeexample.

FIG. 15 is an enlarged sectional view showing a part of a completedsemiconductor device, in particular showing a gate electrode GE in anactive region and an oxide insulating film O1. FIG. 16 is an enlargedsectional view showing a part of a completed semiconductor devicesimilarly to FIG. 15 and is a view explaining the structure by dividingan oxide insulating film O1 integrated by oxidation treatment at thestep explained in reference to FIG. 8 into plural pieces. Here, aninsulating film IF2 over the gate electrode GE, an interlayer insulatingfilm IF3, and a part of a metallic film M1 are not shown in FIGS. 15 and16.

As shown in FIG. 15, the sidewall of a field insulating film FI1 isformed so as to protrude laterally in an eaves-like shape and a part ofa gate electrode GE is embedded into a recess under it. By applying aprescribed potential to the gate electrode GE, even to such a part ofthe gate electrode GE embedded under the eaves-like section, it ispossible to: excite a carrier in a channel; and make a MOSFET Q1on-state normally. Consequently, as long as the gate electrode GE isformed right over the whole upper surface of a body region BR exposed tothe upper surface of an epitaxial layer EP adjacently to a source regionSR, namely right over the whole upper surface of a channel region, witha gate insulating film GF interposed, it is possible to operate theMOSFET without difficulty.

That is, the fact that, right over the whole surface of the channelregion, the terminal of the gate electrode GE in the lateral directionat the part being in contact with the gate insulating film GF existscloser to the body region BR than the boundary between the source regionSR and the body region BR at the upper surface of the epitaxial layer EPcauses the threshold voltage of the MOSFET Q1 to increase. Further, evenwhen the gate electrode GE is formed right over the channel region, theproblem of failing to normally operate the MOSFET Q1 arises if the thickfield insulating film FI1 that is formed continuously from the uppersurface of the epitaxial layer EP is formed and the gate electrode GEunder the eaves-like section is not interposed in between.

This means that, as long as the gate electrode GE comes close right overthe channel region with the gate insulating film GF interposed, noproblem arises even when the gate electrode GE is not formed over thefield insulating film FI1. Consequently, no problem arises even when thegate electrode GE over the upper surface of the field insulating filmFI1 terminates closer to the body region BR than the boundary betweenthe body region BR and the source region SR at the upper surface of theepitaxial layer EP in the direction along the main surface of the SiCsubstrate SB as shown in FIG. 15.

The miniaturization of a semiconductor device is facilitated more as theterminal of the gate electrode GE over the field insulating film FI1comes closer to the end of the upper surface of the field insulatingfilm FI1. The reason is that, if the terminal of the gate electrode GEover the field insulating film FI1 is close to the end of the uppersurface of the field insulating film FI1, it is possible to form acontact region CR and a metallic film M1 that is a contact plug coupledto the contact region CR closer to the gate electrode GE. Further, bybringing the contact plug close to the gate electrode GE in this way,the length of the source region SR between the site where the sourceregion SR and the contact plug are coupled and the channel regionreduces, thus the resistance in the source region SR can be reduced, andthereby the electric power consumption of the semiconductor device canbe reduced.

It is however necessary to prevent the position of the terminal of thegate electrode GE from shifting into a groove T2 because of accuracy inthe forming of the field insulating film FI1, accuracy in the processingof the gate electrode GE, or the like and hence the terminal of the gateelectrode GE is located at a position sufficiently apart from the end ofthe upper surface of the field insulating film FI1 toward the side ofthe contact plug. The distance L in the direction along the main surfaceof the SiC substrate SB between the boundary between the channel regionand the source region SR and the end of the field insulating film FI1having the eaves-like section protruding right over the channel regionis 150 nm for example.

In the present embodiment, since the gate electrode GE is embedded underthe eaves by forming the field insulating film FI1 so as to have the endof an eaves-like inversely-tapered shape, it is possible to bring theterminal of the gate electrode GE over the upper surface of the fieldinsulating film FI1 closer to the center of the groove T2. As a result,it is possible to: reduce the width of the gate electrode GE; bring thecontact region CR and the contact plug closer to the channel region; andhence miniaturize the semiconductor device.

Further, in FIG. 16, insulating films configuring an oxide insulatingfilm O1 are shown separately. A silicon oxide film IF1 having a filmthickness larger than a gate insulating film GF and configuring thebottom part of a field insulating film FI1 is formed beside the gateinsulating film GF. Further, a silicon oxide film OS that is aninsulating film configuring the field insulating film FI1 and is formedby oxidizing a polysilicon film PS1 (refer to FIG. 7) is formed over thesilicon oxide film IF1. The gate insulating film GF is an insulatingfilm formed by partially oxidizing the upper surface of an epitaxiallayer EP by the heat treatment explained in reference to FIG. 8.

The silicon oxide film IF1 is isotropically removed also in the lateraldirection through the wet etching step explained in reference to FIG. 7and hence the end of the silicon oxide film IF1 retracts closer to thecontact plug than the end of the silicon oxide film OS over it. Further,the silicon oxide film OS is formed by oxidizing a polysilicon film PS1(refer to FIG. 7) that is relatively thick and is comprised of silicon(Si) having a high oxidation rate and hence is formed so as to bulge inthe lateral direction further than the polysilicon film PS1 beforeoxidized. Consequently, the end of the field insulating film FI1comprising the silicon oxide films OS and IF1 has a shape protrudinglike an eaves at the sidewall of a groove T2. That is, the sidewall ofthe field insulating film FI1 overhangs right over the gate insulatingfilm GF.

The gate insulating film GF having a film thickness t1 smaller than afilm thickness t2 of the silicon oxide film IF1 is formed in contactwith the upper surface of the epitaxial layer EP right under the end ofthe silicon oxide film OS protruding like an eaves in the lateraldirection. Consequently, the end of the silicon oxide film OS protrudinglike an eaves and the gate insulating film GF right under it areseparated from each other and a part of the gate electrode GE isembedded into a recess formed between them. That is, by reducing thefilm thickness t1 of the gate insulating film GF so as to be smallerthan the film thickness t2 of the silicon oxide film IF1, it is possibleto make the sidewall of the field insulating film FI1 have a morelargely protruding eaves-like shape. As a result, it is possible tomaterialize the configuration of embedding a part of the gate electrodeGE under the eaves section of the sidewall of the field insulating filmFI1.

At the steps explained in reference to FIGS. 1 to 13, the explanationshave been made on the premise that the film thickness of the gateinsulating film GF is 50 nm for example and the film thickness of thesilicon oxide film IF1 (refer to FIG. 5) is 20 nm for example. Even insuch a configuration, the polysilicon film PS1 protruding like an eavesin FIG. 7 bulges in the lateral direction through a succeeding oxidationstep and hence the eaves-like sidewall of the field insulating film FI1is formed as shown in FIG. 15. The magnitude of the protrusion of thesidewall of the field insulating film FI1, namely the width of the gateelectrode GE embedded under the eaves section, can be increased furtherby applying wet etching for a longer period of time for example at thestep explained in reference to FIG. 7.

In contrast, if you want to embed the gate insulating film GF closer tothe silicon oxide film IF1 right under the eaves section, it isconceivable to form the gate insulating film GF and the silicon oxidefilm IF1 having the relationship of t1<t2 as shown in FIG. 16. That is,the film thickness of the silicon oxide film IF1 may be increased tolarger than 50 nm for example in order to materialize the configurationshown in FIG. 16. Otherwise, it is also possible to reduce the filmthickness t1 of the gate insulating film GF and increase the filmthickness t2 of the silicon oxide film IF1 so as to be larger than t1.

A manufacturing method of a semiconductor device, namely a method offorming a field insulating film only from an insulating film stackedover an epitaxial layer, and the problem are hereunder explained as acomparative example. FIGS. 21, 22, 23, and 24 used in the followingexplanations are sectional views, corresponding to FIGS. 5, 7, 8, and13, in the manufacturing steps of a semiconductor device according to acomparative example.

In the comparative example, firstly the steps similar to the stepsexplained in reference to FIGS. 1 to 4 are applied.

Successively, as shown in FIG. 21, a silicon oxide film IF4 having afilm thickness of about 250 nm for example is formed over an epitaxiallayer EP by a CVD method for example.

Successively, as shown in FIG. 22, a pattern of a photoresist film PR4is formed over the silicon oxide film IF4, wet etching is applied withthe photoresist film PR4 used as a mask, thereby the silicon oxide filmIF4 is opened, and thus the upper surface of the epitaxial layer EP ispartially exposed. As a result, an active region where a gate electrodeis embedded later and a field region on both the sides are specified.

At the wet etching step, since the silicon oxide film IF4 of a largefilm thickness is removed isotropically from the parts exposed at thebottoms of the openings of the photoresist film PR4, the sidewalls ofthe silicon oxide film IF4 under the photoresist film PR4 take a taperedshape. That is, the silicon oxide film IF4 has a trapezoidal shape ofdecreasing the width in the lateral direction from the lower surfacetoward the upper surface. In other words, the width of each of theopenings of the silicon oxide film IF4 increases toward the upper part.The silicon oxide film IF4 is an insulating film that is to be a fieldinsulating film and the sidewalls do not take an eaves-likeinversely-tapered shape unlike the present embodiment.

Further, at the wet etching step, since the silicon oxide film IF4 of alarge film thickness is removed isotropically from the upper surface,the magnitude of the retraction of the silicon oxide film IF4 in thelateral direction increases comparatively. In the case where themagnitude of the retraction increases because the film thickness of thesilicon oxide film IF4 processed by wet etching is large, the positionalvariation of the terminal of the silicon oxide film IF4 processed by wetetching intrinsically having a low processing accuracy increasesfurther. Here, since to form the silicon oxide film IF4 that is a fieldinsulating film right over the channel regions adjacent to the sourceregions SR causes the malfunction of a MOSFET, it is necessary toprevent: the position of the terminal of the silicon oxide film IF4processed by wet etching having a low processing accuracy from varying;and the channel regions and the silicon oxide film IF4 from overlappingwith each other.

In order to secure an allowance to the variation, it is necessary toform the ends of the silicon oxide film IF4 so as to be separatedlargely from the boundaries between the channel regions and the sourceregions SR toward the side of the source regions SR, respectively.

Furthermore, at the wet etching step, the sidewall of the silicon oxidefilm IF4 takes a tapered shape and the area of the upper surface of thesilicon oxide film IF4 comes to be smaller than the area of the lowersurface of the silicon oxide film IF4. Consequently, the joint strengthbetween the upper surface of the silicon oxide film IF4 and thephotoresist film PR4 over it weakens and there is the danger that thephotoresist film PR4 peels off from the upper surface of the siliconoxide film IF4 during the wet etching step. On this occasion, since theexposed upper surface of the silicon oxide film IF4 retracts also by thewet etching, an arising problem is that the silicon oxide film IF4cannot secure a film thickness necessary as a field insulating film.

It is necessary therefore to expand the width of the silicon oxide filmIF4 in order to prevent the photoresist film PR4 from peeling off. Onthis occasion, since the widths in the lateral direction of the sourceregions SR and the body regions BR right under the silicon oxide filmIF4 have also to be expanded, another arising problem is that the areaof the semiconductor device increases.

Successively, as shown in FIG. 23, after the photoresist film PR4 isremoved, a gate insulating film GF is formed over the upper surface ofthe epitaxial layer EP in the active region exposed from the siliconoxide film IF4 by applying heat treatment. As a result, an oxideinsulating film O3 including the gate insulating film GF and the siliconoxide film IF4 being formed on both the sides and having a filmthickness larger than the gate insulating film GF is formed.

On this occasion, a recess is formed over the upper surface of the oxideinsulating film O3 at the boundary between the gate insulating film GFand the silicon oxide film IF4, those being in contact with each other.The film thickness of the oxide insulating film O3 right under therecess is smaller than the film thickness of the gate insulating film GFat the center section of the active region. The recess is caused by thefact that the silicon oxide film formed over the upper surface of theepitaxial layer EP in the vicinity of the silicon oxide film IF4 havinga tapered shape is not formed thicker than the silicon oxide film formedover the upper surface of the epitaxial layer EP at the center sectionof the active region.

Successively, as shown in FIG. 24, a semiconductor device according tothe comparative example including MOSFETs Q1 is formed by applying thesteps similar to the steps explained in reference to FIGS. 9 to 13. Onthis occasion, parts of the gate electrode GE are embedded into therecesses at both the ends of the upper surface of the gate insulatingfilm GF. In this case, since the gate electrode GE is formed excessivelyclose to the source regions SR, the insulating film having a lowwithstanding voltage exists locally between the gate electrode GE andthe epitaxial layer EP and that causes the insulation breakdown of thegate insulating film GF at the sites where the recesses are formed. Ifthe gate insulating film GF is destroyed thereby, the semiconductordevice cannot be used anymore.

Further, at the step of forming the gate electrode GE by processing thepolysilicon film (the step corresponding to FIG. 10), it is necessary toterminate the gate electrode GE over the field insulating film having afilm thickness necessary to easily process the gate electrode GE. Inthis regard, since the film thickness of the sidewalls of the siliconoxide film IF4 having a tapered shape is small, it is necessary to avoidthe termination of the gate electrode GE right over the tapered partsfrom the viewpoint of processing the gate electrode GE easily.Consequently, it is necessary to terminate the gate electrode GE atuppermost parts of the tapered parts of the sidewalls of the siliconoxide film IF4, namely at positions sufficiently separated from the endsof the upper surfaces of the silicon oxide film IF4, in the directionaway from the active region right under it.

Thus in the semiconductor device according to the comparative example,the area occupied by elements increases, the miniaturization and higherintegration of a semiconductor chip are hardly attained, and hence thereis the problem of deteriorating the performance of the semiconductordevice. The problem is caused by the fact that it is necessary to formthe ends of the silicon oxide film IF4 away from the channel regionsbecause the sidewalls of the silicon oxide film IF4 are tapered, thusthe width of the silicon oxide film IF4 increases, and the positionswhere the sidewalls of the silicon oxide film IF4 are formed varylargely. Further, the problem is further caused by the fact that it isnecessary to expand the width of the silicon oxide film IF4 in order toprevent the photoresist film PR4 from peeling off and the positionswhere the gate electrode GE terminates get away from the active regionbecause the silicon oxide film IF4 is tapered.

Further, to increase the width of the silicon oxide film IF4 as statedabove means to increase the width in the lateral direction of each ofthe source regions SR interposed between the body region BD that is tobe a channel region and the contact region CR under it. Since each ofthe source regions SR is a semiconductor layer having a high resistancevalue, the increase of the width of the source region SR causes theresistance of a MOSFET to increase. Consequently, in the semiconductordevice according to the comparative example, there is the problem ofdeteriorating the performance of the semiconductor device by increasingthe electric power consumption of the MOSFET.

By a manufacturing method according to the present embodiment incontrast, unlike the comparative example, as explained in reference toFIGS. 5 to 7, the silicon oxide film IF1 having a film thickness smallerthan the polysilicon film PS1 is opened by a wet etching method with thepattern of the polysilicon film PS1 processed by a dry etching methodused as a mask and thereby the active region where the gate insulatingfilm is formed is specified. Here, since the upper surface of theepitaxial layer EP is exposed through the wet etching step (refer toFIG. 7), it is possible to prevent the upper surface of the epitaxiallayer EP from being damaged.

At the wet etching step, since the silicon oxide film IF1 having asmaller film thickness in comparison with the comparative example isprocessed, it is possible to reduce the magnitude and the variation ofretraction of the silicon oxide film IF1 even when a wet etching methodof a low processing accuracy is used. That is, it is possible to improvethe accuracy of the wet etching. Consequently, it is unnecessary to formthe ends of the silicon oxide film IF1, namely the ends of the fieldinsulating film FI1, largely away from the boundaries between thechannel regions and the source regions SR of the MOSFETs toward the sideof the source regions SR in consideration of the low accuracy of the wetetching. Further, since it is unnecessary to take the variation of theopening positions of the field insulating film FI1 caused by the lowaccuracy of the wet etching into consideration, it is possible to bringthe positions of the terminals of the gate electrode GE closer to theactive region. As a result, it is possible to miniaturize thesemiconductor device.

Further, since the film thickness of the silicon oxide film IF1 is muchsmaller than that of the polysilicon film PS1 that is to be the fieldinsulating film together with the silicon oxide film IF1 at a succeedingstep, it is possible to prevent the sidewalls of the silicon oxide filmIF1 from being formed in a tapered shape. As a result, it is possible toprevent the width of the MOSFETs from increasing because the sidewallsof the thick silicon oxide film IF4 (refer to FIG. 22) that is the fieldinsulating film is tapered like the comparative example.

Furthermore, at the step explained in reference to FIG. 7, since thewidth of the upper surface of the silicon oxide film IF1 processed by awet etching method never comes to be extremely smaller than the width ofthe lower surface, it is possible to prevent the film formed over thesilicon oxide film IF1 and used as a mask at the wet etching step,namely the polysilicon film PS1, from being likely to peel off.

In addition, since an insulating film having a sufficiently large filmthickness cannot be formed in the field region only by the silicon oxidefilm IF1, in the present embodiment, the field insulating film FI1having a film thickness larger than the gate insulating film GF shown inFIG. 8 is formed by forming the polysilicon film PS1 having a filmthickness larger than the silicon oxide film IF1 over the silicon oxidefilm IF1 and apply heat treatment to it. That makes it possible, in theoxide insulating film O1, to: form the gate insulating film GF of asmall film thickness in the active region; and form the field insulatingfilm FI1 of a large film thickness in the field region.

In this way, it is possible to specify the active region with a highdegree of accuracy by combining dry etching to process the polysiliconfilm PS1 and wet etching to process the silicon oxide film IF1.Consequently, it is possible to terminate the gate electrode GE that hasto be overlapped with the channel regions in a planar view, namely thegate electrode GE being in contact with the gate insulating film GF inthe active region, at desired positions with a high degree of accuracy.As a result, it is possible to reduce the distance from the boundarybetween each of the source regions SR and the relevant channel region tothe relevant end of the gate electrode GE being in contact with the gateinsulating film GF. It is thereby possible to: reduce the width of theMOSFETs Q1; and hence improve the performance of the semiconductordevice.

A semiconductor device according to the present embodiment manufacturedby the aforementioned method has the following effects.

That is, as shown in FIG. 13, the sidewalls of the field insulating filmFI1 do not have such a tapered shape as to increase the opening width ofthe field insulating film FI1 upward and the ends of the fieldinsulating film FI1 adjacent to the active region have a film thicknessnecessary for easily processing the gate electrode GE. As a result, itis unnecessary to terminate the gate electrode GE at positionssufficiently apart from the active region and it is possible toterminate the gate electrode GE in the vicinities of the ends of thefield insulating film FI1. In other words, it is possible to move theends of the gate electrode GE closer to the center of the gate electrodeGE. Further, since the sidewalls of the field insulating film FI1 do nothave such a tapered shape as the comparative example, it is possible toprevent the width of the field insulating film FI1 from increasing.

Further, the gate electrode GE embedded under the eaves sections of thesidewalls of the field insulating film FI1 can be used for normallyturning the MOSFETs Q1 to an on-state by applying a prescribedpotential. Furthermore, in consideration of the processing accuracy ofthe gate electrode GE explained in reference to FIG. 10, all one need todo is to place the terminals of the gate electrode GE at positions tosome extent apart from the ends of the upper surface of the fieldinsulating film FI1. As a result, as shown in FIG. 15, each of theterminals of the gate electrode GE right over the field insulating filmFI1 is not necessarily located closer to the field region than therelevant end of the active region, the relevant end of the gateinsulating film GF, the boundary between the relevant channel region andthe relevant source region SR, or the end of the part of the gateelectrode GE embedded under the relevant eaves section.

As shown in FIG. 15, the gate electrode GE over the field insulatingfilm FI1 terminates right over the channel regions. That is, the gateelectrode GE over the upper surface of the field insulating film FI1 mayterminate in the regions closer to the active region than the sourceregions SR in the direction along the main surface of the SiC substrateSB. As a result, it is possible to reduce the width of the whole gateelectrode GE.

When the width of the eaves-like shape of the sidewalls of the fieldinsulating film FI1 protruding from the ends of the bottom surface ofthe field insulating film FI1 in the lateral direction is small, thegate electrode GE terminates in the field region, but on this occasiontoo it is possible to reduce the width of the gate electrode GE incomparison with the comparative example, and hence it is possible toreduce the distance from each of the boundaries of the active region andthe field region to the relevant end of the gate electrode GE. As aresult, it is possible to: reduce the width of the gate electrode GE;and miniaturize the MOSFETs Q1 even when the width of the region betweentwo source regions SR adjacent to each other with the relevant channelregion interposed, namely the width of the active region, is not changedin comparison with the comparative example.

Further, it is possible to arrange the contact regions CR and thecontact plugs comprising the metallic film M1 at positions close to theactive region to the extent of reducing the width of the gate electrodeGE as stated above. As a result, it is possible to reduce the width ofthe field region between two parts of the gate electrode GE adjacent toeach other.

In this way, it is possible to: reduce the area occupied by elements;miniaturize and highly integrate the semiconductor chip; and henceimprove the performance of the semiconductor device.

Further, as stated above, it is possible to: arrange the contact regionsCR and the contact plugs comprising the metallic film M1 at positionsclose to the active region; and hence reduce the width of each of thesource regions SR ranging from the end of the source region SR coupledto the relevant contact plug to the end of the source region SR on theopposite side, namely the end of the source region SR being in contactwith the relevant channel. As a result, it is possible to: reduce theelectric current route in the source regions SR having a high resistancevalue; and hence reduce the power consumption of the MOSFETs Q1.Consequently, it is possible to improve the performance of thesemiconductor device.

Further, since the sidewalls of the field insulating film FI1 shown inFIG. 13 have an inversely-tapered eaves-like shape, it is possible toprevent recesses from forming over the upper surface of the gateinsulating film GF adjacent to the sidewalls at the bottoms of thegrooves T2. That is, since it is possible to keep the film thickness ofthe whole gate insulating film GF uniform, it is possible to prevent:the film thickness of the ends of the gate insulating film GF fromreducing; and withstanding voltage from reducing locally between thegate electrode GE and the epitaxial layer EP unlike the comparativeexample. As a result, it is possible to: improve the withstandingvoltage of the MISFETs Q1; and hence improve the performance of thesemiconductor device.

Furthermore, it is possible to improve the performance of thesemiconductor device by embedding the lower sections of the gateelectrode GE under the eaves sections of the sidewalls of the fieldinsulating film FI1, thus fixing the gate electrode GE, making the gateelectrode GE hardly peel off from above the epitaxial layer EP, andincreasing the mechanical strength of the gate electrode GE.

Embodiment 2

A configuration of changing the angle of the eaves-like sidewalls of thefield insulating film in a MOSFET explained in Embodiment 1 so as to bemore vertical to the main surface of a substrate is explained hereunderin reference to FIGS. 17 to 20. FIGS. 17 to 20 are sectional viewsexplaining the manufacturing method of a semiconductor device accordingto the present embodiment.

In the manufacturing step of a semiconductor device according to thepresent embodiment, firstly the steps similar to those explained inreference to FIGS. 1 to 7 are applied. That is, as shown in FIG. 7,after a laminated film is formed by forming a silicon oxide film IF1 anda polysilicon film PS1 in sequence over an epitaxial layer EP over anSiC substrate SB, the laminated film is opened and the upper surface ofthe epitaxial layer EP is exposed.

Successively, as shown in FIG. 17, a polysilicon film PS3 is formed overthe epitaxial layer EP so as to cover the silicon oxide film IF1 and thepolysilicon film PS1 by a CVD method for example. The polysilicon filmPS3 is a semiconductor film comprising a silicon (Si) film having anoxidation rate larger than silicon carbide (SiC). The film thickness ofthe polysilicon film PS3 is 10 nm for example and hence the polysiliconfilm PS3 never completely fills grooves T1 that are openings of thepolysilicon film PS1. The polysilicon film PS3 covers the upper surfaceand the sidewalls of the polysilicon film PS1 and further covers theupper surface of the epitaxial layer EP under the grooves T1 and thesidewalls of the silicon oxide film IF1.

Further, the polysilicon film PS3 completely fills recesses right underthe ends of the polysilicon film PS1 protruding closer to the centers ofthe grooves T1 than the sidewalls of the silicon oxide film IF1. Thatis, the polysilicon film PS3 is embedded between the ends of thepolysilicon film PS1 and the upper surface of the epitaxial layer EP. Asa result, the sidewalls of the polysilicon film PS3 in the grooves T1 donot take an eaves-like shape and are formed at an angle closer to anangle vertical to the upper surface of the epitaxial layer EP. Thepolysilicon film PS3 can completely fill the recesses as long as thefilm thickness of the polysilicon film PS3 is not less than a half ofthe film thickness of the silicon oxide film IF1.

Successively, as shown in FIG. 18, the polysilicon films PS1 and PS3 areoxidized by applying heat treatment similarly to the step explained inreference to FIG. 8. As a result, in an active region, the polysiliconfilm PS3 being in contact with the upper surface of the epitaxial layerEP is oxidized and a gate insulating film GF of about 50 nm in thicknessis formed. Further, in a field region interposing the active region, afield insulating film FI2 including a silicon oxide film formed byoxidizing the polysilicon films PS1 and PS3 and a silicon oxide film IF1(refer to FIG. 17) is formed. The gate insulating film GF and the fieldinsulating film FI2 are integrated with each other and configure anoxide insulating film O2.

Grooves T3 are formed at positions corresponding to the grooves T1(refer to FIG. 17) between adjacent parts of the field insulating filmFI2 and the gate insulating film GF is formed at the bottoms of thegrooves T3. The sidewalls of the field insulating film FI2 have aninversely-tapered shape but are formed at an angle closer to an anglevertical to the upper surface of the epitaxial layer EP than thesidewalls of the field insulating film FI1 (refer to FIG. 8) explainedin Embodiment 1.

This is because the gaps under the eaves sections at the ends of thepolysilicon film PS1 are filled with the polysilicon film PS3 at thestep explained in reference to FIG. 17. Here, at the oxidation step byheat treatment, since the silicon oxide film formed by oxidizing thewhole polysilicon film PS1 having a relatively large film thicknessexpands in the lateral direction, the sidewalls of the grooves T3 takean inversely-tapered shape. That is, the width of the grooves T3 reducesupward.

Successively, as shown in FIG. 19, by applying steps similar to thesteps explained in reference to FIGS. 9 and 10, the patterns of a gateelectrode GE and an insulating film IF2 are formed over the activeregion and the oxide insulating film O2 in the vicinity. The gateelectrode GE fills the grooves T3 and is in contact with the uppersurface of the gate insulating film GF. That is, the gate electrode GEis embedded between the ends of the field insulating film FI2 formed inan eaves-like shape and the gate insulating film GF right under it.

Successively, as shown in FIG. 20, by applying steps similar to thesteps explained in reference to FIGS. 11 to 13, a semiconductor deviceaccording to the present embodiment having a plurality of MOSFETs Q1including the gate electrode GE is formed. In the present embodiment,since the ends of the field insulating film FI2 have aninversely-tapered shape, it is possible to obtain effects similar toEmbodiment 1.

Further, the sidewalls of the field insulating film FI2 according to thepresent embodiment are formed in a direction closer to the directionvertical to the upper surface of the epitaxial layer EP thanEmbodiment 1. As a result, by the heat treatment, it is possible to formthe gate insulating film GF having a more uniform film thickness overthe upper surface of the epitaxial layer EP at the bottoms of thegrooves T3. Further, since the gate insulating film GF is formed byapplying oxidation treatment not to the upper surface of the epitaxiallayer EP but to the deposited polysilicon film PS3 (refer to FIG. 17)here, it is possible to: form the gate insulating film GF having auniform film thickness; and prevent recesses from being formed at theends of the gate insulating film GF. As a result, it is possible to:increase the withstanding voltage of the MOSFETs Q1; thus improve theperformance of the semiconductor device; and prevent the insulationbreakdown of the gate insulating film GF.

Here, when the eaves sections of the sidewalls of the field insulatingfilm are formed so as to protrude largely toward the active regions, theparts of the gate electrode embedded right under the eaves sections havea small film thickness and take an acicular shape of an acute angle in asectional view. A problem arising on this occasion is that an electricfield concentrates on the parts of the gate electrode embedded rightunder the eaves sections and insulation breakdown tends to occur.Further in this case, it is concerned that a mechanical stressconcentrates on the parts of the gate electrode embedded right under theeaves sections and the gate electrode is destroyed.

In the present embodiment in contrast, the sidewalls of the fieldinsulating film FI2 are formed at an angle close to the verticaldirection and hence it is possible to prevent an electric field fromconcentrating on the gate electrode GE right under the eaves sections.As a result, it is possible to: prevent insulation breakdown fromoccurring; increase the withstanding voltage of the MOSFETs Q1; andhence improve the performance of the semiconductor device. Further,since the sidewalls of the field insulating film FI2 are formed at anangle close to the vertical direction, it is possible to prevent astress from concentrating on the gate electrode GE right under the eavessections. As a result, it is possible to: increase the strength of thegate electrode GE; and hence improve the performance of thesemiconductor device.

Although the invention established by the present inventors hasheretofore been explained concretely on the basis of the embodiments, itgoes without saying that the present invention is not limited to theembodiments and can be modified variously within the scope not departingfrom the tenor of the present invention.

What is claimed is:
 1. A manufacturing method of a semiconductor device comprising the steps of: (a) providing a substrate of a first conductivity type containing silicon carbide; (b) forming a semiconductor layer of the first conductivity type containing silicon carbide and having a first region and a second region adjacent to each other along the upper surface thereof over the substrate; (c) forming a first semiconductor region of a second conductivity type different from the first conductivity type over the upper surface of the semiconductor layer in the first region and forming a second semiconductor region of the first conductivity type over the upper surface of the semiconductor layer in the second region; (d) after the step (c), sequentially forming a first insulating film and a first semiconductor film including a material having a larger oxidation rate than silicon carbide over the semiconductor layer; (e) exposing the upper surface of the first insulating film by opening the first semiconductor film in the first region; (f) exposing the upper surface of the first semiconductor region in the first region by opening the first insulating film exposed from the first semiconductor film by a wet etching method; (g) after the step (f), forming a gate insulating film over the semiconductor layer in the first region by oxidation treatment and forming a second insulating film containing a film formed by oxidizing the first semiconductor film and the first insulating film and having a film thickness larger than the gate insulating film by the oxidation treatment; and (h) embedding and forming a gate electrode between plural parts of the second insulating film opposing to each other with the first region interposed therebetween.
 2. A manufacturing method of a semiconductor device according to claim 1, wherein, at the step (f), the sidewall of the first insulating film retracts from the sidewall of the first semiconductor film in the direction along the upper surface of the semiconductor layer.
 3. A manufacturing method of a semiconductor device according to claim 1, wherein the first semiconductor film formed in the step (d) has a film thickness larger than the first insulating film.
 4. A manufacturing method of a semiconductor device according to claim 1, wherein the manufacturing method further includes a step of: (i) forming a contact plug electrically coupled to the first semiconductor region and the second semiconductor region over the semiconductor layer beside the gate electrode.
 5. A manufacturing method of a semiconductor device according to claim 1, wherein the gate electrode terminates right over the second insulating film.
 6. A manufacturing method of a semiconductor device according to claim 1, further including, after the step (f) and before the step (g), a step of: (j) forming a second semiconductor film to fill the gap between the first semiconductor film and the semiconductor layer right under it.
 7. A manufacturing method of a semiconductor device according to claim 1, wherein the second semiconductor region and the substrate configure the source region and the drain region of a field effect transistor including the gate electrode respectively.
 8. A manufacturing method of a semiconductor device according to claim 1, wherein the film thickness of the first insulating film formed in the step (d) is larger than the film thickness of the gate insulating film.
 9. A semiconductor device comprising: a substrate of a first conductivity type containing silicon carbide; a semiconductor layer of the first conductivity type formed over the substrate; a first semiconductor region of a second conductivity type that is different from the first conductivity type and a second semiconductor region of the first conductivity type, those being formed adjacently to each other over the upper surface of the semiconductor layer; a gate electrode formed right over the first semiconductor region through a gate insulating film; and an insulating film formed so as to have a film thickness larger than the gate insulating film right over the second semiconductor region, wherein a part of the gate electrode is located right under the end of the insulating film and is embedded right over the gate insulating film.
 10. A semiconductor device according to claim 9, wherein the minimum angle formed between the upper surface of the gate insulating film and the sidewall of the insulating film is smaller than 90 degrees.
 11. A semiconductor device according to claim 9, wherein the gate electrode terminates right over the insulating film.
 12. A semiconductor device according to claim 9, wherein a contact plug electrically coupled to the first semiconductor region and the second semiconductor region is formed over the semiconductor layer beside the gate electrode. 